Large structural components which are part of larger structures and which must be fabricated to close dimensional tolerances are typically assembled on hard tooling and fastened together by hand. This entails high labor costs and continual calibration and maintenance of the fixturing tooling, which can experience very hard usage in the factory. An example of such a structure is an airplane wing spar, which is a structural component of an airplane wing that couples the upper and lower wing skin and provides stiffness to the wing. Each wing has a forward and a rear spar, each bent to accommodate the swept configuration of the wing. As a consequence, there are four unique spars on each airplane, each of which would normally require its own tooling and associated periodic calibration and maintenance.
The cost of manual assembly of large mechanical structures can be considerable, especially when the structure must be built to very close dimensional tolerances. The assembly procedure requires highly skilled labor and often requires rework when stringent quality requirements, such as exist in the airframe industry are not met by some rivet or other fastener. Moreover, the hand assembled structures require long assembly times which can increase the number of tooling sets required when high volume production is needed.
The parts to be assembled to make the structure should be easily loaded onto the assembly machine and be positionable thereon with great accuracy and speed. It is probably necessary to provide a fine adjustment on the machine to ensure that the parts are located thereon at the exactly correct position, within the tolerances required by the manual process, including all the routine ones such as rivet insertion and threading nuts on bolts. All these processes must be performed with speed and precision and must be repeatable for thousands of cycles without fault to avoid the need for time consuming operator intervention.
Flexibility is a desirable attribute of an automated assembly machine. In the event that one of the machines requires service, it would be a valuable feature if one of the machines could be reconfigured temporarily, and quickly, to enable the assembly of the structure normally made on another machine on that machine, thereby maintaining the necessary flow of completed structures to the factory.